Thermophoretic collection and analysis of submicrometer Ag particles emitted from a graphite tube-type electrothermal vaporizer

Abstract

Using thermophoretic collection with a cooled Cu probe, particles generated in an electrothermal vaporizer (ETV) from a AgNO3 solution sample have been collected and analyzed using transmission electron microscopy and high-energy electron diffraction (HEED). Ag particles are spherical and, interestingly, have diameters falling into one of four size regimes, <5, 15, 25, and >40 nm. An extremely large number (estimated at > 108) of particles with diameters of < 100 nm are collected. For an aqueous AgNO3 solution deposited in the ETV, the HEED pattern of the collected aerosol particles exiting the pulse heated ETV matched body centered cubic Ag(s), and the large number of diffraction spots suggests that particles are composed of microcrystalline domains. The features of the particles confirm earlier predictions of homonucleation as the primary particle formation mechanism. 'Groupings' of apparently disconnected particles were a unique feature seen from the pulse-heated vaporization of the dried sample. This morphology is unique and does not appear like any other clustering or aggregation of particles reported elsewhere. It is not clear what causes this particle grouping, although the absence of these groupings when a 700 °C thermal pretreatment step was introduced suggests that the AgNO3 decomposition plays a role in their formation. It is suggested that the particles formed from homonucleation are created very near the graphite surface and are cooled quite rapidly to form the solid silver particles. A mechanism is presented to explain the appearance of silver in the gas phase at temperatures below the vaporization temperature for silver metal.